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The mileage is about 80 thousand km, which is equipped with Citroen 0L with 2. 0L engine and manual transmission. User response: after running for a period of time, the cooling fan can not be stopped for a long time. Fault analysis: check the external condition of engine without obvious abnormal phenomena. The diagnostic system is used to diagnose the engine system without fault code. Start the engine, check the data flow, and find that when the engine temperature reaches 97 degrees Celsius, the cooling fan starts to operate at low speed. Then when the engine temperature drops to 89 C, as long as the engine does not extinguish, the fan is still running at low speed, and the cooling fan should stop running normally. Combined with the structural characteristics of the car, the factors affecting the operation of the fan have three aspects: the engine temperature, the oil temperature of the automatic transmission and the refrigerant pressure of the air conditioning system. Maintenance work should start from these three aspects, the specific maintenance process is as follows: 1) check the oil temperature of the automatic transmission. The automatic transmission system is self diagnosed by using a diagnostic instrument without any fault information. Start the engine and carry out the road test, and check the parameters of the system at the same time, confirm that the oil temperature of the automatic transmission is kept at about 100 degrees C, and the temperature requirement of the cooling fan is not reached (130 degrees C). The automatic transmission oil temperature sensor and its circuit are inspected normally, thus eliminating the fault factors of too high oil temperature of the automatic transmission. 2) check the coolant temperature sensor and its circuit. Check the liquid level of coolant, within the standard range. Check engine temperature sensor and its circuit, normal. Replacement of engine temperature sensor, failure still exists. 3) check the low speed relay of the cooling fan. It is no problem to remove the relay for inspection. Check the connection of the line, normal. 4) check the engine control module. If there is a certain malfunction in the engine system, the engine control module will control the fan running at low speed in order to prevent the engine from overheating. Therefore, it is necessary to check the engine control module. No obvious anomalies were found in the actual inspection results. Replace the engine control module, match it, trial run, the fault still exists. 5) check the refrigerant pressure sensor. First, understand the working characteristics of the Picasso air conditioning system: when the refrigerant pressure sensor (linear pressure sensor) measures the refrigerant pressure to reach 1000kPa, the engine control module will control the relay 1508, and at this time the PSF1 (the fuse box in the engine compartment) provides the working power to the low speed resistor. The cooling fan operates at low speed. Therefore, refrigerant pressure sensor is the focus of inspection. As a result of actual inspection, the refrigerant pressure sensor has normal performance. 6) check the refrigerant pressure. Connecting the air conditioning pressure gauge to measure the static pressure of the refrigerant, the pressure of the low pressure side is 1100kPa, the pressure of the high pressure is 1180kPa, and the two pressure is obviously higher than the normal pressure. It seems that the fault is caused by the high pressure of the refrigerant. The refrigerant is recovered, the refrigerant is re - quantitatively added (700g), and the refrigerant static pressure is measured, normal. Carry out road test, when the engine temperature reaches 97 degrees, the cooling fan runs at low speed. When the engine temperature drops to 89 degrees, the cooling fan stops. At this point the fault is completely eliminated. Picasso 2. 0L automatic air conditioning system is more advanced. The system can centrally control the cooling air and heating in the vehicle and realize ventilation, heating and refrigeration integration. The components of the electrical control part mainly include air conditioning switch, air conditioner compressor relay, evaporator temperature sensor, coolant temperature sensor, refrigerant pressure switch, engine control module, intelligent control box and so on. The working conditions of the fan are as follows: when the engine temperature rises to 97 degrees C, the fan begins to operate at low speed; opens the air conditioning switch and the fan is running at low speed; when the engine temperature reaches 105, the cooling fan runs at a high speed; when the refrigerant pressure is more than 1600kPa, the cooling fan is quickly transported in the fan, and when the pressure of the refrigeration system is large At 2100kPa, the cooling fan operates at a high speed; when the refrigerant pressure is greater than 2600kPa, the circuit of the compressor is disconnected; when the refrigerant pressure is less than 250kPa, the circuit of the compressor is disconnected; when the engine temperature reaches 115 C, the circuit of the compressor is disconnected; when the engine temperature reaches 118 degrees C, the dashboard alarm. The vehicle has only one radiator fan, and its working condition is controlled by the engine control module, the intelligent control box and the refrigerant pressure switch. In combination with this case, the refrigerant pressure sensor transfers this information to the engine control module because the refrigerant pressure of the air conditioning system pipeline is too high. In accordance with the related control program (the refrigerant pressure is too high, the cooling fan is controlled to operate at low speed so that the system pressure can be reduced to protect the components. " The engine control module makes the terminal 2 of relay 1508 grounded, and the cooling fan runs at low speed. However, because the refrigerant pressure is always too high, the cooling fan is running at low speed. After reinjection of refrigerant, the system pressure is restored to normal value, and the trouble is completely eliminated. The failure of this example shows the importance of the quantitative injection refrigerant. Many air conditioning failures are related to the cooling dose, so the cooling dose directly affects the refrigerant pressure and the load of the air conditioning system.